U.S. Pat. No. 4,700,081 discloses a speed avoidance logic for a variable speed wind turbine, wherein the generator torque (or generator power) is controlled in a manner which maneuvers the rotor tip speed with respect to the wind speed. In particular, a torque command signal (or power reference) is provided to the converter for controlling the power flow to the power grid and hence for controlling the air gap torque in the AC generator for controlling the generator torque.
US 2009/0292397 A1 discloses a method and an apparatus for damping tower oscillation in a wind turbine, wherein the rotor rotational speed is controlled such that a critical rotor speed is avoided. In particular, the power reference is increased such that the power reference and the rotor speed both follow the optimum curve again at the point where the rotational speed is leaving the critical window.
It has however been observed that the conventional method for controlling a rotor frequency may not be accurate enough, may not work in certain conditions, or may even not be possible (in particular during low wind conditions, or during curtailment).
There may be a need for a method and a system for determining a pitch angle offset signal for controlling a rotor frequency of a rotor of a wind turbine and for a method for controlling a rotor frequency of a rotor of a wind turbine, wherein at least some of the above-mentioned problems have been overcome. In particular, there may be a need for a method and a system for determining a pitch angle offset signal and for a method for controlling a rotor frequency of a rotor of a wind turbine, wherein a problems regarding a coincidence of a resonance frequency of an oscillation mode of the wind turbine and a rotor frequency or a harmonic of the rotor frequency are reduced.